Merge pull request #6248 from ethereum/shiftopt-fix-overflow

Fixes u256 overflow in logical shift optimization rule and adds tests.

docs/bugs.json

@@ -1,4 +1,16 @@
 [
+    {
+        "name": "DoubleShiftSizeOverflow",
+        "summary": "Double bitwise shifts by large constants whose sum overflows 256 bits can result in unexpected values.",
+        "description": "Nested logical shift operations whose total shift size is 2**256 or more are incorrectly optimized. This only applies to shifts by numbers of bits that are compile-time constant expressions.",
+        "introduced": "0.5.5",
+        "fixed": "0.5.6",
+        "severity": "low",
+        "conditions": {
+            "optimizer": true,
+            "evmVersion": ">=constantinople"
+        }
+    },
     {
         "name": "ExpExponentCleanup",
         "summary": "Using the ** operator with an exponent of type shorter than 256 bits can result in unexpected values.",

docs/bugs.rst

@@ -52,9 +52,15 @@ severity
     discoverability in contract tests, likelihood of occurrence and
     potential damage by exploits.
 conditions
-    Conditions that have to be met to trigger the bug. Currently, this
-    is an object that can contain a boolean value ``optimizer``, which
+    Conditions that have to be met to trigger the bug. The following
+    keys can be used:
+    ``optimizer``, Boolean value which
     means that the optimizer has to be switched on to enable the bug.
+    ``evmVersion``, a string that indicates which EVM version compiler
+    settings trigger the bug. The string can contain comparison
+    operators. For example, ``">=constantinople"`` means that the bug
+    is present when the EVM version is set to ``constantinople`` or
+    later.
     If no conditions are given, assume that the bug is present.
 check
     This field contains different checks that report whether the smart contract

docs/bugs_by_version.json

@@ -630,7 +630,9 @@
         "released": "2019-02-12"
     },
     "0.5.5": {
-        "bugs": [],
+        "bugs": [
+            "DoubleShiftSizeOverflow"
+        ],
         "released": "2019-03-05"
     }
 }